This invention relates to an improved process for recovering P.sub.2 O.sub.5 values contained in the hemihydrate crystals and unreacted calcium phosphate rock generated in the hemihydrate process for manufacturing phosphoric acid. The process of this invention is less dependent on external variables than prior art processes, and is more efficient both in terms of recovery time and yield.
The phosphoric acid, which is primarily used for agricultural purposes, is commonly manufactured today using the so-called "wet process". This process involves the processing of mined phosphate rock by solubilization with phosphoric acid and reaction with sulfuric acid to produce a phosphoric acid solution and insoluble calcium sulfate. In the dihydrate process, the phosphoric acid produced has a P.sub.2 O.sub.5 concentration of from about 26% to about 30%, and the insoluble calcium sulfate is present in the dihydrate form (CaSO.sub.4.2H.sub.2 O). The hemihydrate process results in a phosphoric acid product having a P.sub.2 O.sub.5 concentration of from about 38% to about 50% and an insoluble calcium sulfate product which is in the hemihydrate form (CaSO.sub.4.sup..1/2H.sub.2 O). Further details of both processes are disclosed in U.S. Pat. Nos. 4,132,760, 4,196,172 and 4,220,630.
Since phosphoric acid concentrations of about 40% on a P.sub.2 O.sub.5 basis or above are usually required for the production of phosphate fertilizer products, the hemihydrate process has a significant advantage over the dihydrate process in being capable of directly producing phosphoric acid within this concentration range. However, the hemihydrate process also has a number of significant limitations which appear to be inherent in the process, such as poor slurry filterability, low product recovery, cake conversion on the filter, scaling, etc. These limitations prevented the early commercialization of the hemihydrate process. As a result, the dihydrate process gained early acceptance in the industry, and became the conventional process.
Over the passage of time, the continuous rise in energy costs increased the incentive for the development of the more energy efficient hemihydrate process. Eventually, several commercial variations of the hemihydrate process emerged after overcoming some of the initial process limitations, and this started a trend in the phosphate industry toward the hemihydrate mode of operation.
Although economics generally favors the hemihydrate process over the dihydrate process, the hemihydrate process is not without its own inherent limitations as previously mentioned. One particular disadvantage of the hemihydrate process is recovery which can be 2% to 3% lower than in the dihydrate process. This lower recovery is due primarily to the more pronounced occlusion of P.sub.2 O.sub.5 values in the calcium sulfate filter cake. These so-called "lattice bound losses" are due to several factors such as the substitution of phosphate in the crystal lattice of the hemihydrate crystals, the entrapping of phosphoric acid inside of the polycrystals formed during the hemihydrate process, and the formation of Al--F--P.sub.2 O.sub.5 complexes on the hemihydrate crystal surfaces. Lattice bound losses increase with conditions that favor replacement of sulfate with phosphate, such as a higher product acid concentration and a lower free sulfate level in the phosphoric acid.
One way to recover the occluded P.sub.2 O.sub.5 values, as well as the P.sub.2 O.sub.5 values trapped in the unreacted calcium phosphate rock, is to recrystallize the hemihydrate crystals as dihydrate crystals. In this recrystallization process, the hemihydrate filter cake, which is discharged from the hemihydrate filter, is fed to a dihydrate phosphoric acid plant, or to a recrystallization section, where the hemihydrate crystals are dissolved and recrystallized as dihydrate crystals. This releases the occluded P.sub.2 O.sub.5 from the crystals along with any other impurities normally retained in the filter cake. The liquid phase, which is separated from the dihydrate crystals during filtration, is phosphoric acid which normally has a concentration of 12% to 18% on a P.sub.2 O.sub.5 basis. This filtrate, or dihydrate acid, is used to wash the hemihydrate filter cake on a hemihydrate filter. The resulting second filtrate from the hemihydrate filter is blended with product acid to form a recycle acid stream having a concentration in the range of 33% to 40% on a P.sub.2 O.sub.5 basis. This recycle acid is returned to the hemihydrate reactor to dissolve the phosphate rock feed.
The hemihydrate slurry which is formed in the hemihydrate reactor is filtered on the hemihydrate filter to separate the filter cake from a phosphoric acid product normally having a concentration in the range of 40% to 46% on a P.sub.2 O.sub.5 basis.
Theoretically, the use of a recrystallization stage to recrystallize calcium sulfate dihydrate should lead to the production of a stronger phosphoric acid product, i.e. one having a P.sub.2 O.sub.5 concentration of 44% to 50%, as compared to the single stage hemihydrate process which can have a somewhat lower P.sub.2 O.sub.5 concentration. The stronger acid should result in a larger amount of lattice-bound P.sub.2 O.sub.5 formed during the crystallization, and this additional lattice-bound P.sub.2 O.sub.5 should be recovered in the recrystallization stage and should eventually appear in the final acid product. In practice, however, due to the conditions required for recrystallization, optimum recovery is obtained at lower acid concentrations, usually in the range of 43% to 45% on a P.sub.2 O.sub.5 basis.
The operation of the recrystallization stage is critical for recovery of P.sub.2 O.sub.5 values since the rate of recrystallization decreases sharply with increasing concentration of the dihydrate acid. For instance, at a dihydrate acid concentration above about 20% on a P.sub.2 O.sub.5 basis, the rate of recrystallization is too slow, resulting in incomplete recrystallization and low yield. This result could, if serious enough, render the process inoperable. In contrast, if the dihydrate acid concentration is too low, i.e. 12% on a P.sub.2 O.sub.5 basis or less, a large recycle of the product acid would be required to satisfy the product phosphoric acid concentration requirements. This would reduce the overall process throughput, making the operation less efficient and more costly.
This interdependence of the hemihydrate and dihydrate sections of the process is a serious disadvantage, requiring the synchronization of both the hemihydrate and dihydrate processes which is not always possible. A realignment of both processes for synchronization necessitates additional down-time, adversely affecting the overall operating efficiency of the process, the actual recovery, and the overall process economics. In addition, a recrystallization scheme, such as outlined above, requires several hours for complete conversion of hemihydrate crystals to dihydrate crystals, and this is a significant disadvantage in any commercial process.